The present embodiments relate to a method for producing a gradient coil assembly for a magnetic resonance imaging system.
In a magnetic resonance imaging system, an object to be examined may be exposed with the aid of a basic field magnet magnetic system to a relatively high basic field magnet magnetic field (e.g., 3 or 7 tesla). A magnetic field gradient is also applied with the aid of a gradient system. High frequency excitation signals (HF signals) are emitted via a high frequency transmission system using suitable antenna devices, and the HF signals are intended to tilt the nuclear spin of certain atoms excited by the high frequency field about a defined flip angle with respect to the basic magnetic field. The high frequency excitation and the resulting flip angle distribution may also be referred to as “core magnetization” or “magnetization” below. During relaxation of the nuclear spin high frequency signals, magnetic resonance signals that are received by suitable receiving antennae and are processed further are radiated. The desired image data may be reconstructed from the raw data thus acquired. The high frequency signals may be emitted for nuclear spin magnetization using a “whole body coil” (e.g., a “body coil”). The whole body coil may be constructed in the manner of a hollow cylinder around a patient space of the magnetic resonance imaging system. An object to be examined or a patient is positioned in the hollow cylinder for an examination using a moveable examination table. One construction for this is a birdcage antenna. The birdcage antenna includes a plurality of transmitting rods that are arranged so as to run parallel to a longitudinal axis around the patient space of the magnetic resonance imaging system. At an end face, the antenna rods are annularly capacitively connected to each other, respectively.
The gradient system for producing the magnetic field gradient includes, for example, a hollow cylinder-shaped gradient coil assembly surrounding the whole body coil. The gradient coil assembly includes at least a plurality of primary gradient coils and may also include secondary gradient coils. The primary gradient coils are the coils, with which the desired gradient field is produced in the patient space, while the secondary gradient coils are used to outwardly shield the magnetic field produced by the primary gradient coils (e.g., the secondary gradient coils are shielding coils). A gradient coil assembly may include three primary gradient coils in order to be able to apply a magnetic field gradient in the direction of the x, y axes and z axis (e.g., the longitudinal axis of the patient space) independently of each other. The gradient coil assembly may also include corresponding separate secondary gradient coils for the x, y and z axes, respectively. The gradient coil assembly may include a shim coil assembly with shim coils (e.g., field coils of a higher order), with which the magnetic fields may be readjusted. To produce a gradient coil assembly of this kind, the individual gradient coils and optionally the shim coils are applied one after the other to a winding mandrel in the correct sequence from the inside out and are fixed to each other (e.g., cast). Once the winding mandrel has been removed, the finished gradient coil arrangement may be measured in a field measuring device. The coupling inductance between the individual coils of the various gradient axes and optionally relating to the shim coils is determined in addition to the field characteristic of the gradient coil. Instances with excessive coupling inductance between the individual coils or undesirable field differences may be rejected.
To produce the magnetic resonance imaging system, the finished whole body coil and the finished gradient coil arrangement are arranged such that the finished whole body coil and the finished gradient coil arrangement are pushed into each other in the basic field magnet.
With this construction, the size of the patient space, in which a patient is situated during the examination, is defined by the internal diameter of the whole body coil. The diameter of the patient space may be about 60 cm. The consequence of this is that many patients feel claustrophobic during such examinations. The diameter of the patient space may be increased. To achieve an enlargement, the basic field magnet may be enlarged. This, however, leads to significant cost increases, as the magnetic fields become ever stronger. Alternatively, the components of the magnetic resonance imaging system (e.g., the gradient coil assembly and the whole body coil) arranged in the basic field magnet around the patient space may have dimensions reduced in the radial direction. To increase the patient space from 60 cm to 70 cm, for example, the radial dimensions of the ensemble of the gradient coil assembly and the whole body coil is reduced from a thickness of about 150 mm to about 100 mm. For example, the thickness of the gradient coil assembly is reduced from about 100 mm to 70 mm. The sensitivity of the signal field of the gradient coils to manufacturing variations increases significantly due to this reduction in space, so only a low manufacturing tolerance is acceptable. Consequently a higher reject rate is expected.